This project will investigate the development of discourse in adolescents who have mental retardation due to Down syndrome (DS) or fragile X syndrome (FXS). Each syndrome is characterized by impairments in the domains of behavioral functioning that support discourse development (e.g., cognition, receptive language) and, thus, individuals with either syndrome are likely to find discourse to be especially challenging. There are also differences in the behavioral phenotypes of DS and FXS, which may lead to different profiles of discourse problems. There are few data on the discourse skills of individuals with DS or FXS and fewer data still on discourse problems that may be specific to one syndrome or the other. The need for such data is especially acute during the adolescent years. During adolescence, preparation begins for adult roles that demand expertise in discourse. This project will examine discourse in adolescents with DS and adolescents with FXS from a model in which discourse is seen to emerge from the collorative activity of people working toward a common goal. This project has three specific aims. This first is to determine (a) whether discourse development is more severely disrupted in these adolescents than expected from their levels of functioning in domains that support discourse development (i.e., cognition, linguistic ability) and (b) whether it is disrupted differently in the two syndromes. The second aim is to (a) identify the ways in which developments of nonverbal cognition, language, social cognition, speech, and hearing contribute to the ability of these adolescents to collaborate effectively and (b) determine whether these domains make different contributions to discourse for DS and FXS. The third aim is to determine what syndrome-specific impairments (i.e. psychopathology and auditory memory problems) contribute to discourse collaboration. Five studies will address these aims. In each, participants will engage in a referential task that is designed to highlight differences between the syndromes. Comparisons with younger, typically developing children will determine the extent to which discourse development is disrupted in each syndrome. Measures will be taken of domains in which there are syndrome-shared impairments. Hierarchial multiple regression will be used to examine the contributions of these domains to the discourse collaboration of the DS and FXS groups.